1. Field of the Invention
The present invention relates to a ceramic composition and in particular to a ceramic composition which has a high dielectric constant and insulation resistance, a low rate of variation in dielectric constant with temperature and a low decrease in dielectric constant upon application of a DC bias.
2. Description of the Prior Art
When a multilayer capacitor is produced from a ceramic composition, the ceramic composition should be selected so that it satisfies several requirements. The composition should have a dielectric constant and a specific resistivity as high as possible and that a rate of variation in dielectric constant with temperature, dielectric loss and drop in dielectric constant due to DC bias application should be as low as possible. Among ceramic compositions having high dielectric constants, those mainly comprising barium titanate (BaTiO.sub.3) are well known. However, these compositions require a high sintering temperature. Accordingly, an additive such as calcium titanate (CaTiO.sub.3) or lead titanate (PbTiO.sub.3) is incorporated to improve temperature-dependent characteristics. However the sintering temperature thereof is still on the order of not less than 1300.degree. C. For this reason, when the ceramic composition mainly comprising barium titanate is used in making a multilayer ceramic capacitor, materials for internal electrodes thereof are limited to, for example, noble metals (e.g., platinum and palladium) which can withstand such a high sintering temperature. Further, the dielectric constant achieved by these dielectric ceramic compositions is at highest about 8000. The dielectric constant thereof may be increased, but, at a cost of temperature stability. More specifically, these ceramic compositions simply satisfy Y5V characteristics (-30.degree. to 85.degree. C.; +22%, -82%) as defined in the EIA Standards.
Although a ceramic material showing a low rate of variation in dielectric constant with temperature can be produced from the conventional material, the dielectric constant of the resulting ceramic composition is too low (on the order of about 2000) to use as a material for capacitors.
To reduce the expenses for producing multilayer ceramic capacitors, it is necessary to develop a ceramic composition capable of being sintered at a low temperature on the order of not more than 1150.degree. C., and which uses less expensive materials for internal electrodes of capacitors such as those mainly comprising silver or nickel. Recently, there have been proposed various lead-based composite perovskite type compounds having low sintering temperatures and high dielectric constants. For instance, a ternary composition comprising lead magnesium niobate [Pb(Mg1/3Nb2/3)O.sub.3 ], lead nickel niobate [Pb(Ni1/3Nb2/3)O.sub.3 ] and lead titanate (PbTiO.sub.3) can achieve a dielectric constant at room temperature of not less than 10000 (see U.S. Pat. No. 4,712,156). However, this three-component system suffers from a problem of high temperature-dependency of the dielectric constant. Moreover, the capacitances thereof are greatly reduced upon application of a DC bias and correspondingly, the resulting capacitors are greatly limited in their practical applications.